The invention relates to a method and an apparatus for producing single crystalline diamonds by Microwave Plasma Chemical Vapor Deposition (MPCVD) process. More specifically, it describes a method and an apparatus for producing one or more single crystalline diamonds at a high growth rate by generating high substrate temperatures at low microwave power.
Diamonds are crystalline allotropes of carbon. They are renowned for their superlative physical qualities, especially their hardness and their high dispersion of light. These properties make diamonds valued for use in jewelry and a variety of industrial applications. A natural diamond is formed naturally in the earth due to the prolonged exposure of carbon-bearing materials to high pressure and temperature. Scientists have been able to produce synthetic diamonds under laboratory conditions, which have similar chemical composition and physical properties as natural diamonds.
Deposition of polycrystalline diamond films first attracted the attention of scientists after it was discovered that when a mixture of methane and hydrogen gases is subjected to a Chemical Vapor Deposition (CVD) process, atomic hydrogen prevents the formation of energetically favorable graphite. Research in the field of diamond production got a further impetus with the development of an efficient way of generating atomic hydrogen in a mixture of methane and hydrogen when plasma is created at a microwave frequency of 2.45 GHz. Subsequently, various CVD methods such as hot filament, radio-frequency plasma and arc-jet torch have been used to deposit diamond films using the same principle.
However, the diamond films produced by the above-mentioned methods are polycrystalline in nature and growth rates are typically between 0.5-3 microns per hour. Moreover, the polycrystalline diamond films have a large concentration of grain boundaries that deteriorate the properties of the micron-sized crystallites present in these films. As a result, these films have very limited applications.
Single crystalline diamonds are used as a potential material for fabricating high pressure anvils, electronic devices, optical windows, and heat sink applications, etc. Consequently, producing single crystalline diamonds has become increasingly important. Some of the important parameters for producing single crystalline diamonds using CVD method are: substrate temperature, deposition pressure and microwave power.
U.S. Pat. No. 5,370,912, titled ‘Diamond Film Deposition with a Microwave Plasma’, describes a method for depositing a diamond film using a microwave plasma generating apparatus that includes a tunable microwave cavity. The microwave cavity is tuned by adjusting its height and the depth of the insertion of an antenna. The height of the cavity and the depth of the insertion of the antenna are continuously adjusted to minimize the reflected radiation and vary the shape and position of the plasma. The method involves heating the diamond substrate to about 950° C. to 1,100° C., and creating a plasma, including hydrogen gas, within the deposition chamber at a pressure of at least about 30 torr. Further, the method involves monitoring the plasma to ensure its proper position, and maintaining the plasma for a sufficient time to deposit the desired thickness of the diamond film. However, the diamond films deposited by this method are poly-crystalline in nature.
U.S. Pat. No. 6,858,078, titled ‘Apparatus and Method for Diamond Production’, describes an apparatus and a method for growing a single crystalline diamond by the MPCVD process. The method for producing the single crystalline diamond involves positioning a diamond in a heat-sinking holder such that a thermal contact is made with a side surface of the diamond adjacent to an edge of a growth surface of the diamond. The temperature of the growth surface of the diamond is measured to generate temperature measurements using a non-contact temperature measurement device. Moreover, the method involves controlling temperature of the growth surface, based upon the temperature measurements.
However, the methods and apparatus described above have limitations with respect to generating high substrate temperature in the CVD chamber at low microwave power for producing diamonds. As high substrate temperature is essential for the growth of single crystalline diamonds, a power source of higher microwave power is required. This decreases the efficiency of the method for producing single crystalline diamonds.
In light of the foregoing discussion, there is a need for a method and an apparatus for producing single crystalline diamonds that can generate high substrate temperature at low microwave power. Further, the method and the apparatus should be able to achieve high growth rate while producing single crystalline diamonds. Furthermore, there is a need for a method and an apparatus that maintains uniform growth conditions at the growth surface of substrates when multiple single crystalline diamonds are to be produced.